Soil Organic Carbon and Carbon Dioxide Emission from an Organically Amended Hawaiian Tropical Soil

Application of organic manure (OM) to arable lands improves soil tilth. The objectives of this study were to: (i) simulate the effect of OM application rates (0, 168, 336, and 672 kg total N ha(-1)) and types (chicken [Gallus domesticus] and dairy manures) on soil organic C (SOC) and CO2 emissions from a Hawaiian highly weathered tropical soil; and (ii) correlate SOC, CO2 emissions, and two major soil properties: bulk density (rho(b)) and saturated hydraulic conductivity (K-sat). Measurements of SOC and rho(b) were conducted on samples collected from the top 10 cm of soil tilled before and after manure application, cultivated with sweet corn ('Super Sweet 10,' Zea mays L. ssp. mays), and drip irrigated for two consecutive growing seasons. The K-sat values were calculated from infiltration data measured with a tension infiltrometer. The Rothamsted C turnover model was used to simulate SOC and CO2 emissions. The simulated and measured SOC agreed reasonably during model calibration (r(2) = 0.93) and validation (r(2) = 0.90). Results revealed that SOC, CO2 emissions, and K-sat increased while rho(b) decreased with increasing OM rates. There was no significant effect of OM type. There was a highly significant (P < 0.01) correlation between the measured and simulated SOC and between the measured SOC and the simulated CO2 emissions. The K-sat values significantly correlated (P < 0.05) with the measured and simulated SOC and the simulated CO2 emissions. A significant inverse relationship between rho(b) and K-sat was observed. We concluded that, in addition to improving soil aggregation, decreasing rho(b), and increasing K-sat, OM application to this tropical soil increases SOC pools that contribute to atmospheric CO2 following tillage and other agricultural practices.